Sputter coating is a process carried out in a vacuum chamber which is filled with a generally chemically inert gas in which a substrate is coated with a material for a target of sputtering material subjected to a negative electrical potential with respect to the chamber wall or other anode. The potential gradient adjacent the target surface causes electrons to be emitted from the target which, on their way to the chamber anode which is usually formed in part by the grounded chamber wall, strike and ionize some of the inert gas. The positive ions formed are then attracted to the negative target which they strike, transferring momentum to the target material, and ejecting particles of the material from the target surface. The substrate to be coated, which is positioned in the chamber usually with its surface facing the target, receives some of the ejected particles which adhere to and coat the substrate surface.
With magnetron sputtering, a magnetic field is formed over the target surface, usually including magnetic field lines parallel to the target surface, and, in many applications, in the form of a closed magnetic tunnel. The magnetic field causes the electrons emitted to move in curved spiral paths which trap them in regions proximate the target surface enclosed by the field, thereby increasing the rate of electron collisions with gas atoms, which in turn increase the ionization of the gas and the efficiency of the sputtering process.
In the commonly assigned and copending U.S. patent application Ser. No. 07/339,308, filed Apr. 17, 1989, entitled "Method and Apparatus for Sputter Coating Stepped Wafers", expressly incorporated herein by reference, a sputter coating apparatus and method are disclosed in which a concave annular target is provided with concentric annular electromagnets which cause the formation of a pair of concentric plasma rings. The plasma rings are alternately energized by alternately supplying current to energize the magnet coils while the target power level is switched in synchronization with the switching of the current to the magnetic coils. This causes different rates of sputtering from inner and outer concentric regions of the target surface, with the sputtering from each region causing different distribution characteristics of the sputtered material deposited on the substrate or wafer being coated. Varying the relative parameters affecting the energization of the two target regions provides control of coating uniformity on the substrate surfaces, which is especially important on the differently facing surfaces of stepped semiconductor wafers. The aforereferenced patent application particularly describes effects on the coating caused by the target geometry and by the electrical parameters relating to the energization of the target and plasmas.
In bias sputtering, a voltage which is negative, but less negative than that imposed on the target, is applied to the substrate being coated. This bias voltage causes a certain amount of "back sputtering", or sputtering from the sputter coating which has been deposited on the substrate surface, due to the impingement of ions produced by electrons emitted from the substrate. Frequently, however, particularly where the sputtering target is annular with an annular or other closed magnet trap over its surface, pole pieces behind the target produce magnetic fields which shape "primary" plasmas near the target surface and produce a fringing field in the vicinity of the substrate surface which is non-uniform. Components of the fringing field so produced are perpendicular to the substrate surface in certain regions, as for example, at the center on the axis of the target magnet center pole. This fringing field and other portions of the electric and magnetic fields tend to concentrate regions of "secondary" plasma formation which produces a secondary ion flux bombardment of the substrate. The result is often an undesired non-linear distribution of ion flux on the substrate surface.
A solution to the problem of non-uniform ion flux distribution on the surface of the substrate is described in the commonly assigned U.S. Pat. No. 4,871,433 entitled "Apparatus for Improving the Uniformity of Ion Bombardment In a Magnetron Sputtering System". In that patent, the use of a secondary magnet behind or around the substrate having certain characteristics is described. The secondary magnet, or countermagnet, modifies the fringing field produced by the cathode or target primary magnet to render the ion flux uniform on the substrate. While effective, the apparatus in that patent is somewhat specific to the cathode and cathode magnet arrangement, is large and difficult to fit into many processing chamber arrangements, and often must be replaced when the target magnet scheme is changed. Furthermore, the level of ion flux which is provided with such a countermagnet system is limited.
Certain sputter coating devices of the prior art which employ annular sputtering targets employ the use of an anode at the target center. The anode is provided to avoid heating of the wafer due to secondary electron bombardment of the wafer. Such targets are provided with what is called a dark space shield which surrounds the target at its outer rim to absorb electrons which stray from the plasma, preventing them from striking the substrate surface. The anodes provided at the target centers are maintained at the same grounded or other anode potential of the dark space shield to absorb stray electrons near the target center. Such electrons, if not kept from the substrate, while heating the surface, are thought to collect on the surfaces of layers deposited onto conductive substrates, eventually causing a breakdown of the non-conductive material and damage to the substrate surface. These prior art electrodes were also placed to critically intercept some of the plasma trapping field lines, generally reducing their effectiveness.